FMS6346E Fairchild Semiconductor, FMS6346E Datasheet - Page 10

• Manufacturer Part Number: FMS6346E
• Description: Six-channel, Selectable Sd / Hd Video Filter Driver With Disable
• Manufacturer: Fairchild Semiconductor

© 2010 Fairchild Semiconductor Corporation
FMS6346E • Rev.2.0.10
1.0 -> 1.02V
0.65 -> 0.67V
0.3 -> 0.32V
0.0 -> 0.02V
2.28V
1.58V
0.88V
0.28V
There is a 280mV offset from the DC input level to the
DC output level. V
0.85V
0.5V
0.15V
1.98V
1.28V
0.58V
Applications Information
Functional Description
The FMS6346E Low-Cost Video Filter (LCVF) provides
6dB gain from input to output. In addition, the input is
slightly offset to optimize the output driver performance.
The offset is held to the minimum required value to
decrease the standing DC current into the load. Typical
voltage levels are shown in Figure 10.
The FMS6346E offers three channels with internal
diode clamps and three channels with biasing to
support AC-coupled input signals. If the input signal
does not go below ground, the input clamp does not
operate. This allows DAC outputs to directly drive the
FMS6346E without an AC-coupling capacitor. The
worst-case sync-tip compression due to the clamp does
not exceed 7mV. The input level set by the clamp,
combined with the internal DC offset, keeps the output
within its acceptable range. When the input is AC
coupled, the diode clamp sets the sync-tip (or lowest
voltage) just below ground.
For symmetric signals like C, Pb, and Pr; the average
DC bias is fairly constant and the inputs are biased to
set the DC input voltage to approximately 600mV. DAC
outputs can also drive these same signals without the
AC coupling capacitor. A conceptual illustration of the
input clamp circuit is shown in Figure 11.
Figure 10. Typical Voltage Levels
V
V
V
OUT
OUT
OUT
IN
= 2 • V
V
IN
IN
+ 280mV.
Driven by:
DC-Coupled DAC Outputs
AC-Coupled and Clamped
Y, CV, R, G, B
Driven by:
AC-Coupled and Biased
U, V, Pb, Pr, C
10
0.65V
I/O Configurations
For DC-coupled DAC drive with DC-coupled outputs,
use the configuration shown in Figure 12.
If the DAC’s average DC output level causes the signal
to exceed the range of 0V to 1.4V, it can be AC coupled
as shown in Figure 13.
When driven by an unknown external source or a
SCART switch with its own clamping circuitry, the inputs
should be AC coupled as shown in Figure 14.
External video
source must
be AC coupled
Figure 13. AC-Coupled Inputs, DC-Coupled Outputs
Figure 14. SCART Configuration with DC-Coupled
Y
IN
DVD or
DVD or
Output
Output
DAC
STB
SoC
DAC
STB
SoC
Figure 12. DC-Coupled Inputs and Outputs
Figure 11. Input Clamp Circuit
800k
0V - 1.4V
75Ω
0.1μF
0.1μF
0V - 1.4V
0V - 1.4V
Outputs
Driver
Inactive
Clamp
LCVF
Clamp
Active
LCVF
Clamp
Active
LCVF
Y
OUT
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75Ω
75Ω
75Ω